344 Anulewicz et al.: Emerald Ash Borer Density and Canopy Dieback in Ash ash trees (S 22.0; df 1,7; P > 0.36) (Figure 5). For the two trees of each species that were felled and debarked in 2004, the percentage of phloem surface area covered by gal- leries ranged from 25% to 100% in white ash, with an aver- age of 67% (±5.9 SE), and from 0% to 40% in blue ash, with an average of 15% (±7.6 SE). In 2005, percentage of phloem covered by galleries ranged from 0% to 100% in white ash, with an average of 73% (±8.1 SE), and from 0% to 30% in blue ash, with an average of 7% (±5.0 SE). The percentage of phloem covered by galleries did not differ significantly be- tween species in 2004 (S 7.0; df 1,2; P > 0.17), but were significant in 2005 (S10.0; df1,6; P0.0143). Callous tissue grew around the edge of many of the galleries on blue ash trees, but we observed little evidence of callous tissue on white ash trees. The overall frequency and extent of woodpecker predation was notable; more than 86% of the green, white, and blue ash trees included in our surveys had at least one woodpecker attack. The percentage of A. planipennis killed by wood- peckers on urban street trees ranged from 0% to 100%, with an average of 33% (±5.8 SE). This predation rate was greater for woodlot trees, where woodpecker attacks killed 0% to 100% of the A. planipennis, with an average mortality of 48% (±6.6 SE). DISCUSSION One of the key questions our study addressed was whether A. planipennis exhibits a host preference hierarchy among the predominant ash species native to Michigan. We determined that A. planipennis densities were consistently higher in green ash trees than in white ash trees growing in the same resi- dential sites. Our counts represented the cumulative density of larvae that successfully completed their feeding and either emerged as adults or were preyed on by woodpeckers as prepupae. Our data also illustrate the progression of the A. planipennis infestation in sites monitored for multiple years. In the residential sites, A. planipennis densities in white ash increased from year to year as available phloem in green ash declined. The only exception to this trend was at Windemere in 2005, where, on average, white ash trees had significantly higher exit hole and woodpecker attack densities than green ash both before and after the removal of the outlier point. Unlike the other sites that had fairly equal numbers of each species in the surrounding area, Windemere was unique in that the street trees were predominantly white ash with an occasional green ash tree in the midst of white ash trees. Singer (1983) noted that insects may not use their most highly preferred host if they encounter another acceptable host first. The reduced density of exit holes and woodpecker attacks in the green ash from 2004 to 2005 at Windemere [40 exit holes and woodpecker attacks per meter squared (4 per ft2)] also reflected the removal of two dead green ash trees in 2005. ©2007 International Society of Arboriculture Average A. planipennis densities would have been substan- tially greater if the dead trees had been retained in our annual surveys. Similarly, A. planipennis densities in trees at the Larned site would likely have been higher in 2005 if four dead trees had not been removed in 2004. At the Butzel site, where most trees were less than 5 cm (2 in) dbh, average A. planipennis densities were consistently higher in green ash than white ash trees, but differences between species were not significant. Because the trees were small, only a limited amount of phloem was available for A. planipennis at this site, probably obscuring some interspecific differences. Agrilus planipennis clearly preferred white ash trees over blue ash trees in both woodlots we surveyed. When our sur- vey began in 2004, numerous white ash trees in the two woodlots were declining or had been killed by A. planipennis, whereas nearby blue ash trees were just becoming infested. Blue ash trees may have also been less suitable hosts for developing larvae. We noted that callous tissue often grew on the edges or over the top of larval galleries on blue ash trees but was rarely associated with galleries on white or green ash trees. It was also evident that most larvae would have taken 2 years to develop on the blue ash trees. Future studies are needed to address potential differences among ash species in their phloem chemistry, attraction to ovipositing beetles, and initial resistance to A. planipennis larval feeding. Canopy dieback caused by A. planipennis was consistently more pronounced in green ash trees than white ash trees at all residential sites in all years. Similar patterns have been ob- served in other infested areas. For example, in a residential site in Washtenaw County, Michigan, dieback on green ash trees averaged 60% compared with less than 10% dieback for white ash trees of the same age (McCullough et al. 2005). As expected, we found an increasing proportion of the canopy died each year, because the density of A. planipennis larvae escalated and the proportion of intact phloem declined. Although canopy dieback, an external symptom of infes- tation, is not necessarily a direct measure of larval density, we did find a significant correlation between canopy dieback and the density of A. planipennis exit holes and woodpecker at- tacks. Linear models derived to describe this relationship were nearly identical for green ash and white ash trees, indi- cating that in both species, a 10% increase in dieback was associated with eight or nine more A. planipennis per meter squared (≈1 per ft2). Ash trees and other ring-porous hardwoods are generally more vulnerable to injury caused by phloem-feeding insects or wilt diseases than diffuse-porous hardwoods or conifers (Haack and Benjamin 1982; Kozlowski and Pallardy 1997). In the 14 dead green ash and 19 dead white ash trees we sampled in the residential and woodlot sites, the maximum number of A. planipennis per meter squared that were able to develop on a single tree was ≈107 (±10.8 SE) per meter squared [10 (±1.0 SE) per ft2] for green ash and 109 (±10.6
September 2007
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
